Iran and the International Atomic Energy Agency (IAEA) have failed to agree on the start of an investigation into Tehran's disputed nuclear program.

Iranian officials and international nuclear inspectors met in Vienna on May 15, but could not settle on an agreement that would allow international inspectors access to Iranian nuclear sites, documents, and officials.

After the meeting, IAEA Deputy Director General Herman Nackaerts said the UN watchdog's commitment to continued dialogue remained "unwavering."

No date for a new round was agreed.

Later on May 15 in Istanbul, EU foreign policy chief Catherine Ashton will meet Iran's chief nuclear negotiator Said Jalili.

The meeting between Ashton, who represents six world powers, and Jalili follows a failed round of big-power diplomacy in Kazakhstan in early April.

The six world powers are the United States, Russia, China, France, Britain, and Germany.

The meetings come as the Obama administration says it is looking at new ways to pressure Iran over its nuclear program.

Addressing the Senate Foreign Relations Committee in Washington on May 15, Treasury Under Secretary for Terrorism and Financial Intelligence David Cohen indicated that Washington would "work to target additional sources of Iranian revenue including the petrochemical sector."

"With our colleagues at [the] State [Department], we will maintain a robust outreach effort to foreign governments and the private sector to explain our sanctions, to warn of the risks of doing business with Iran, and to encourage them to take complimentary steps," he said.

"We will continue to aggressively implement and target Iran’s proliferation networks, support for terrorism, sanctions evasion, abuse of human rights and complicit financial institutions, and we will continue to work closely with [the U.S.] Congress in each and every one of these endeavors because we know that we share a common objective -- ensuring that Iran does not obtain a nuclear weapon."

Meanwhile, the United States has blacklisted two financial firms based in the United Arab Emirates for dealing with an Iranian bank blacklisted by Washington.

The Treasury Department said May 15 that Al Hilal Exchange and Al Fida International General Trading had provided financial services to Bank Mellat.

The decision means U.S. citizens and companies are barred from dealing with the two firms.

Any assets held by those companies in the United States are now blocked.